English physicist and mathematician Sir Isaac Newton's second law, usually written as F = ma where F is the force, m is the mass of the object and a is its acceleration, states that the acceleration of a body is proportional to the force applied to it and inversely proportional to its mass. Although the second law is usually written in this form it is not the form that Newton originally used in his Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) nor can it be used in situations where the mass of the object may be changing. The original form was written in terms of momentum and is currently expressed as F = dp/dt where F is the force, dp is the change in momentum and dt is the time interval. Although Newton expressed his second law of motion...